1. Field of the Invention
This invention addresses the filtering of a video signal within a video display or projection device to minimize the visual artifacts produced by the display or projection of an image as a set of pixels. This invention addresses the prefiltering of video signals for a pixel based display or projection device, and the optical postfiltering of the image produced by the display or projection device. A pixel based display or projection device is a device wherein the image is present as an array of discrete picture elements, or pixels. Such displays and projectors are currently used for color television, computer devices, and LCD displays, and their use will continue to increase as flat panel screens and high definition television (HDTV) becomes increasingly prevalent.
2. Discussion of the Relevant Arts
Conventional television video images are produced by sweeping an electronic beam across the screen horizontally, then moving the beam vertically, and sweeping the beam again across the screen horizontally. During this horizontal sweep, the beam is appropriately modulated to produce an image corresponding to the video image to be displayed across this horizontal line. Similar techniques are employed for projecting an image; for ease of language, the term display will be used herein to include both the creation of the image upon the surface of the display device, as well as the creation of the image by the projection of light from the device to another surface.
The display of color images requires a partitioning of the display into discrete "picture elements", or "pixels". Conventionally, each pixel on a color display consists of a triad of red, green, and blue areas, configured as either dots or bands. Three individual electron beams are swept across the screen, appropriately modulated to excite the corresponding red, green, and blue areas to produce the desired image. The resolution of the screen, i.e., the ability to display detail, is determined by the number of individual pixel elements, because any detail occurring between the spacing of the individual pixel elements will not be displayed. That is, the displayed video image is, effectively, a display of the original image as it appears at each pixel element, rather than a continuous display of the original image. As disclosed herein, this partitioning of an image into individual picture elements is, effectively, a sampling of the video image.
In other devices, such as Liquid Crystal Displays (LCDs), plasma displays, etc., this pixel-sampling of the image to be displayed is explicit, and the effects are more severe. These devices have individually addressable picture elements. The luminance of each picture element is set proportional to a sample of the luminance of the image to be displayed corresponding to the location of this picture element. If the display is a color display, the luminance of each color in the picture element is set accordingly. The resolution of such displays is determined by the number of individually addressable elements comprising the display.
Two problems are introduced by this sampling phenomenon:
if the resolution of the display is insufficient, low frequency visual artifacts will be introduced; and, PA1 if the discrete samples are not appropriately filtered, high frequency visual artifacts will be produced.
In addition to being visually disturbing, the introduction of visual artifacts which are not present in the original image may have severe consequences in such applications as medical image processing, because the image will show artifacts which are not really present in the item being examined.
It is well known in the art that the information to be displayed must be bandwidth limited so as not to exceed the capabilities of the communications channel across which the information travels. Anti-aliasing filters are conventionally applied to the video image before it is encoded for transmission, and reconstruction filters are employed in the receiver to accurately reproduce this filtered video image. This anti-alias filtering and reconstruction, however, is conventionally applied without regard to the capabilities of the specific device upon which the image is to be created. In conventional displays, signals which exceed the device's ability to be displayed are merely suppressed by the device's insufficient sampling. For example, a common computer video format is 1024 pixels per horizontal line, and conventional monitors are available with 0.28 mm and 0.39 mm dot pitch. A 0.28 mm dot pitch is approximately equivalent to 90 pixels per inch; a 0.39 mm dot pitch is approximately equivalent to 65 pixels per inch. A 15" monitor's width is about 12 inches, which equates to 1080 pixels per horizontal line for a 0.28 mm pitch display, but only 780 pixels per horizontal line for a 0.39 mm pitch display. Images of 1000 pixels per line, however, are typically communicated to such 780 pixel per line displays. Attempting to display the 1000 pixels to a 12 inch, 0.39 mm dot pitch, display will not only result in a loss of resolution, but, more significantly, as disclosed herein, will result in the introduction of signals not present in the original image. That is, absent this invention, if the display has insufficient resolution, not only is the finer detail lost, as expected, but new features, or artifacts, will be introduced into the displayed image.
Even with a display having sufficient resolution to display the finer detail, the discrete display of individual pixel elements, with discontinuities between adjacent pixels, introduces high frequency harmonics which are also not present in the original image. As disclosed herein, the display of an image on a pixel based display is, effectively, a sampling process, yet conventional displays do not contain the reconstructive filters necessary to accurately reproduce the original image. For example, it is known that an image having fine detail, equivalent to about 500 cycles of changes per horizontal line can be displayed on a monitor having over 1000 pixels per horizontal line. However, the actual image will typically be continuous, whereas the displayed image is composed of discrete samples of this image. A common prior art technique for reducing the visual interference caused by the display of discrete elements is to purposely "blur" the image, so that the edge between pixels becomes less visible. Another technique for improving pixel images is dithering, wherein a pixel's content is affected by a purposeful spill-over from adjacent pixels, also with the intent of blurring the edges between pixels. Each of these techniques, however, operate by distorting the image, rather than by a purposeful reconstructive filtering of the displayed image samples.
Additionally, a discrete pixel display, such as an LCD, operates by utilizing a sample-and-hold technique. The characteristic of the image is sampled, then applied to the components forming the pixel. This characteristic of the image is applied for the entire extent of the pixel. As is well known in the art of sampling, the frequency response of a sample-and-hold device results in an attenuation of the original signal which increases with frequency. That is, the sample-and-hold characteristic of a conventional discrete pixel display introduces an undesirable attenuation of the original image, particularly at the higher frequencies.